From anyplace on Earth, the clearest thing in the night sky is usually the moon, Earth's only natural satellite and the nearest celestial object (240,250 miles or 384,400 km away). Ancient cultures revered the moon. It represented gods and goddesses in various mythologies -- the ancient Greeks called it "Artemis" and "Selene," while the Romans referred to it as "Luna."

When early astronomers looked at the moon, they saw dark spots that they believed were seas (maria) and lighter regions that they believed was land (terrae). Aristotle's view, which was the accepted theory at the time, was that the moon was a perfect sphere and that the Earth was the center of the universe. When Galileo looked at the moon with a telescope, he saw a different image -- a rugged terrain of mountains and craters. He saw how its appearance changed during the month and how the mountains cast shadows that allowed him to calculate their height. Galileo concluded that the moon was much like Earth in that it had mountains, valleys and plains. His observations ultimately contributed to the rejection of Aristotle's ideas and the Earth-centered universe model.

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Because the moon is so close to the Earth relative to other celestial objects, it's the only one to which humans have traveled and set foot upon. In the 1960s, the United States and Russia were involved in a massive "space race" to land men on the moon. Both countries sent unmanned probes to orbit the moon, photograph it and land on the surface. In July 1969, American astronauts Neil Armstrong and Edwin "Buzz" Aldrin became the first humans to walk on the moon. During six lunar landing missions from 1969 to 1972, a total of 12 American astronauts explored the lunar surface. They made observati­ons, took ­photographs, set up scientific instruments and brought back 842 pounds (382 kilograms) of moon rocks and dust samples. What did we learn about the moon from these historic journeys?

Let's take a closer look at the moon. We'll examine its surface features and learn about its geology, internal structure, phases, formation and influence on the Earth.

What's on the surface of the moon?

As we mentioned, the first thing that you'll notice when you look at the moon's surface are the dark and light areas. The dark areas are called maria. There are several prominent maria.

Mare Tranquilitatis (Sea of Tranquility): where the first astronauts landed

Mare Imbrium (Sea of Showers): the largest mare (700 miles or 1100 kilometers in diameter)

Mare Serenitatis (Sea of Serenity)

Mare Nubium (Sea of Clouds)

Mare Nectaris (Sea of Nectar)

Oceanus Procellarum (Ocean of Storms)

The maria cover only 15 percent of the lunar surface.

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The remainder of the lunar surface consists of the bright highlands, or terrae. Highlands are rough, mountainous, heavily cratered regions. The Apollo astronauts observed that the highlands are generally about 4 to 5 km (2.5 to 3 miles) above the average lunar surface elevation, while the maria are low-lying plains about 2 to 3 km (1.2 to 1.8 miles) below average elevation. These results were confirmed in the 1990s, when the orbiting Clementine probe extensively mapped the lunar surface.

The moon is littered with craters, which are formed when meteors hit its surface. They may have central peaks and terraced walls, and material from the impact (ejecta) can be thrown from the crater, forming rays that emanate from it. Craters come in many sizes, and you'll see that the highlands are more densely cratered than the maria.

Another type of impact structure is a multi-ringed basin. These structures were caused by huge impacts that sent shockwaves outward and pushed up mountain ranges. The Orientale Basin is an example of a multi-ringed basin.

Besides craters, geologists have noticed cinder cone volcanoes, rilles (channel-like depressions, probably from lava), lava tubes and old lava flows, which indicate that the moon was volcanically active at some point.

The moon has no true soil because it has no living matter in it. Instead, the "soil" is called regolith. Astronauts noted that the regolith was a fine powder of rock fragments and volcanic glass particles interspersed with larger rocks.

Upon examining the rocks brought back from the lunar surface, geologists found the following characteristics:

The maria consisted primarily of basalt, an igneous rock derived from cooled lava.

The highland regions include mostly igneous rocks called anorthosite and breccia

If you compare the relative ages of the rocks, the highland areas are much older than the maria. (4 to 4.3 billion years old versus 3.1 to 3.8 billion years old).

The lunar rocks have very little water and volatile compounds in them (as if they've been baked) and resemble those found in the Earth's mantle.

The oxygen isotopes in moon rocks and the Earth are similar, which indicates that the moon and the Earth formed at about the same distance from the sun.

The density of the moon (3.3 g/cm3) is less than that of the Earth (5.5 g/cm3), which indicates that it doesn't have a substantial iron core.

Astronauts placed other scientific packages on the moon to collect data:

Seismometers didn't detect any moonquakes or other indications of plate tectonic activity (movements in the moon's crust)

Magnetometers in orbiting spacecraft and probes didn't detect a significant magnetic field around the moon, which indicates that the moon doesn't have a substantial iron core or molten iron core like the Earth does.

Let's look at what all of this information tells us about the formation of the moon.

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Think you know all there is to know about the moon? Test your knowledge with our Moon Quiz.

Giant Impactor Hypothesis

According to the Giant Impact hypothesis, about 4.45 billion years ago a Mars-size body slammed into the young Earth. It melted and merged into the core of the Earth, and the resulting debris coalesced to form the moon.

At the time of Project Apollo in the 1960s, there were basically three hypotheses about how the moon formed.

Double planet (also called the condensation hypothesis): The moon and the Earth formed together at about the same time.

Capture: The Earth's gravity captured the fully formed moon as it wandered by.

Fission: The young Earth spun so rapidly on its axis that a blob of molten Earth spun off and formed the moon.

But based on the findings of Apollo and some scientific reasoning, none of these hypotheses worked very well.

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If the moon did form alongside the Earth, the composition of the two bodies should be about the same (they aren't).

The Earth's gravity isn't sufficient to capture something the size of the moon and keep it in orbit.

The Earth can't spin fast enough for a blob of material the size of the moon to just spin off.

Because none of these hypotheses was satisfactory, scientists looked for another explanation.

In the mid-1970s, scientists proposed a new idea called the Giant Impactor (or Ejected Ring) hypothesis. According to this hypothesis, about 4.45 billion years ago, while t­he Earth was still forming, a large object (about the size of Mars) hit the Earth at an angle. The impact threw debris into space from the Earth's mantle region and overlying crust. The impactor itself melted and merged with the Earth's interior, and the hot debris coalesced to form the moon.

The Giant Impactor hypothesis explains why the moon rocks have a composition similar to the Earth's mantle, why the moon has no iron core (because the iron in the Earth's core and impactor's core remained on Earth), and why moon rocks seem to have been baked and have no volatile compounds. Computer simulations have shown that this hypothesis is feasible.

Moon Facts

Distance from Earth: 240,250 miles (384,400 km)

Diameter: 2,160 miles (3,476 km), or about 27 percent of the Earth’s diameter

Mass: 7.35 x 1022 kilograms, about 1.2 percent of the Earth’s mass

Gravity: 1.62 m/s2, or 16.6 percent of the Earth’s gravity

Mean surface temperature:

sunlight = 266 F (130 C),

­shadow = -292 F (-180 C)

Atmosphere: None

Orbital period: 29.5 day

Lunar day: 29.5 Earth days (the moon is tidally locked to the Earth, so our gravity drags the moon around on its axis and the same side of the moon always faces Earth)

Geologic History of the Moon

­Based on analyses of the rocks, crater densities and surface features, geologists came up with the following geologic history of the moon:

After the impact (about 4.45 billion years ago), the newly formed moon had a huge magma ocean over a solid interior.

As the magma cooled, iron and magnesium silicates crystallized and sank to the bottom. Plagioclase feldspar crystallized and floated up to form the anorthosite lunar crust.

Later (about 4 billion years ago), magma rose and infiltrated the lunar crust, where they reacted chemically to form the basalt. The magma ocean continued to cool, forming the lithosphere (which is like the material in the Earth's mantle). As the moon lost heat, the asthenosphere (the next layer in) shrank toward the core and the lithosphere became very large. These events led to a model of the moon's interior that is very different from that of the Earth. Lunar Behavior The moon is thought to influence our daily life and moods, possibly even causing odd behavior. In fact, it's the inspiration for the word "lunatic." Werewolf aficionados, of course, know that a full moon triggers terrifying transformations. And hospital and emergency personnel tell of more crimes, accidents and births during a full moon -- but the evidence for this is mostly anecdotal rather than statistical.

From about 4.6 to 3.9 billion years ago, the moon was intensely bombarded by meteors and other large objects. These impacts modified the lunar crust and gave rise to the large, densely cratered surface in the lunar highlands. Some of these bombardments produced large, multi-ringed basins and mountains.

When the bombardment ceased, lava flowed from the inside of the moon through volcanoes and cracks in the crust. This lava filled the maria and cooled to become the mare basalts. This period of lunar volcanism lasted from about 3.7 billion years to 2.5 billion years ago. Much of the moon's heat was lost during this period. (Because the moon's crust is slightly thinner on the side that faces the Earth, lava could erupt more easily to fill the maria basins. This explains why there are more maria on the near side of the moon compared to the far side.)

O­­nce the volcanic period ended, most of the moon's internal heat was gone, so there was no major geologic activity -- meteor impacts have been the only major geologic factor at work on the moon. These impacts have not been as intense as in earlier periods of the moon's history; bombardments have generally been declining throughout the solar system. However, the meteoric bombardment that continues today has produced some large craters on the maria (like Tycho and Copernicus) and the fine regolith (soil) that covers the lunar surface.

Let's look at some of the phenomena involving the moon's orbit.

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Moon Phases

2008 HowStuffWorks

Every night, the moon shows a different face in the night sky. On some nights we can see its entire face, sometimes it's partial, and on others it isn't visible at all. These phases of the moon aren't random -- they change throughout the month in a regular and predictable way.

As the moon travels in its 29-day orbit, its position changes daily. Sometimes it's between the Earth and the sun and sometimes it's behind us. So a different section of the moon's face is lit up by the sun, causing it to show different phases.

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Over the billions of years of the moon's existence, it has moved farther away from the E­arth, and its rate of rotation has also slowed. The moon is tidally locked with the Earth, which means that the Earth's gravity "drags" the moon to rotate on its axis. This is why the moon rotates only once per month and why the same side of the moon always faces the Earth.

Tides

Every day, the Earth experiences tides, or changes in the level of its oceans. They're caused by the pull of the moon's gravity. There are two high tides and two low tides every day, each lasting about six hours.

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The moon's gravitational force pulls on water in the oceans and stretches the water out to form tidal bulges in the ocean on the sides of the planet that are in line with the moon. The moon pulls water on the side nearest it, which causes a bulge toward the moon. The moon pulls on the Earth slightly, which drags the Earth away from the water on the opposite side, making another tidal bulge there. So, the areas of the Earth under the bulge experience high tide, while the areas on the thin sides have low tide. As the Earth rotates underneath the elongated bulges, this creates high and low tides about 12 hours apart.

The moon also stabilizes the Earth's rotation. As the Earth spins on its axis, it wobbles. The moon's gravitational effect limits the wobble to a small degree. If we had no moon, the Earth might move almost 90 degrees off its axis, with the same motion that a spinning top has as it slows down.

Precession

As the Earth spins on its axis, it wobbles like a top, and the axis slowly moves in the direction that it points. This movement (which is called precession) has occurred slowly over a period of 26,000 years, and it has caused the position of the Earth's pole star to change. Polaris is the current North Star, but in 3000 B.C., it was Thuban, in the constellation Draco. In A.D. 14,000, Vega in the constellation Lyra will be the pole star.

Return to the Moon

Since 1972, no one has set foot on the moon. However, there is a renewed effort for a lunar return. Why? In 1994, the orbiting Clementine probe detected radio reflections from shadowed craters at the moon's South Pole. The reflections were consistent with the presence of ice. Later, the orbiting Lunar Prospector probe detected hydrogen-rich signals form the same area, possibly hydrogen from ice.

Where could water on the moon have come from? It was probably carried to the moon by the comets, asteroids and meteors that have impacted the moon over its long history. Water was never detected by the Apollo astronauts because they didn't explore that region of the moon. If there is indeed water on the moon, it could be used to support a lunar base. The water could be split by electrolysis into hydrogen and oxygen -- the oxygen could be used to support life and both gases could be used for rocket fuel. So, a lunar base could be a staging point for future exploration of the solar system (Mars and beyond). Plus, because of the moon's lower gravity, it is cheaper and easier to lift a rocket off of its surface than from Earth.

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It might tricky to get back there though, at least for U.S. astronauts. In 2010, President Barack Obama decided to cancel the Constellation program, the intent of which was to get Americans back on the moon by 2020. That means U.S. astronauts may have to hitch a ride with private space companies, which will receive some funding from NASA.

Other countries, including Japan and China, are planning to travel to the moon and researching how to build a lunar base using materials from the lunar surface. Various plans call for heading to the moon and establishing possible bases between 2015 and 2035.

To learn more about the moon, take a look at the links on the next page.

Lunar Eclipses

In its orbit, the moon passes between the sun and the Earth and then behind the Earth. Relative to the Earth-sun plane, the moon's orbit is slightly inclined by about 3 degrees. Occasionally, the exact alignment of sun, moon and Earth produces a solar eclipse. This happens only when the moon is in the new phase and its orbit intersects the Earth-sun plane between the Earth and the sun. The moon blocks out the sun and its shadow passes over the Earth.

In the same month as a solar eclipse, when the moon is full, a lunar eclipse will occur. In a lunar eclipse, the moon passes through the Earth’s shadow, which causes its light to dim. If the moon passes through the part of the Earth’s shadow called the umbra, a total lunar eclipse will occur. If the moon passes only through the penumbra, a partial lunar eclipse will occur.